I have a 335i. For those of you who dont know its a 3.0l i6 that comes stock with twin turbos. As far as the current tuning is concerned i have to run dual bank O2 sensors which means i have to run pre turbo sensors since i will be running a single turbo. To solve this problem i am going to make my manifold look something like this

http://www.n54tech.com/forums/attach...9&d=1355986568

however i would like to have it completely separated all the way up to the divided t4 flange. This way i can run dual O2 sensors and benefit from a twin scroll manifold. I was going to do that and run a collecting 60mm wastegate that branches off from both sides and is split right up until the gate. My question has to do with the pipe size that would flow best. The car has 1.25 inch primaries and i was wondering what sizing i should go from there. I was thinking 3 1.25 inch primary banks leading to 2 2 inch pipes which would make their way up to the turbo but i dont know if dual 2 inch is over kill and would even fit on the flange. Any advice on the piping size and even how i route my piping would be much appreciated as im a bit of a newbie in the world of fabrication.

 

That should be fine.

 

What kind of boost will you be running? I ran 2" primaries on mostanifolds up to now. Really I haven't seen a reason to go down. As for fitting on the flange, anything bigger than that, and it will be hard to fit them on a t4 flange.

 

How do you guys weld those and take shrinking into account? My first manifold was a log manifold and it fit perfect and all tacked up. Took it off the flange and welded it and not it isn't even close to fitting.

 

boost will be under 30 psi should be making 700+whp. Schedule 40 1 1/4 inch pipe has an ID of 1.38 and the 2 inch has 2.067 ID in my mind 2 inch just seems a bit big i would like nice spool but at the same time the next size down is 1 1/2 which i think will be too restrictive

 

shrinking isn't that bad if fitment is great... when your tacking runners together tack one side and then the other, I do four tacks 12 oclock then 6 then 9 and 3 and then when you start your root or if you do single pass just go an inch or so and then switch sides.. in theory everything should stay put.

 

I was doing a log manifold, tacked it all together. Took it off the flange and welded and then it shrank.